As disclosed in U.S. Patent Application Publication No. 2006/0258718 A1, carbamic acid (R)-1-aryl-2-tetrazolyl-ethyl esters (hereinafter referred to as “the carbamate compounds”) possess anticonvulsant activity and are useful in the treatment of disorders of the central nervous system, especially including anxiety, depression, convulsion, epilepsy, migraines, bipolar disorder, drug abuse, smoking, ADHD, obesity, sleep disorders, neuropathic pain, strokes, cognitive impairment, neurodegeneration, strokes and muscle spasms.
Depending on the position of N in the tetrazole moiety thereof, the carbamate compounds are divided into two positional isomers: tetrazole-1-yl (hereinafter referred to as “1N tetrazole”) and treatzole-2-yl (hereinafter referred to as “2N tetrazole”). The introduction of tetrazole for the preparation of the carbamate compounds results in a 1:1 mixture of the two positional isomers which are required to be individually isolated for pharmaceutical use.
Having chirality, the carbamate compounds must be in high optical purity as well as chemical purity as they are used as medications. In this regard, U.S. Patent Application Publication No. 2006/0258718 A1 uses the pure enantiomer (R)-aryl-oxirane as a starting material, which is converted into an alcohol intermediate through a ring-opening reaction by tetrazole in the presence of a suitable base in a solvent, followed by introducing a carbamoyl group into the resulting alcohol intermediate. For isolation and purification of the 1N and 2N positional isomers thus produced, column chromatography is utilized after the formation of an alcohol intermediate or carbamate.
For use in the preparation described above, (R)-2-aryl-oxirane may be synthesized from an optically active material, such as substituted (R)-mandelic acid derivative via various route, or obtained by asymmetric reduction-ring formation reaction of α-halo arylketone, or by separation of racemic 2-aryl-oxirane mixture into its individual enantiomers. As such, (R)-2-aryl-oxirane is an expensive compound.
In addition, the ring-opening reaction of (R)-2-aryl-oxirane with tetrazole is performed at relatively high temperatures because of the low nucleophilicity of the tetrazole. However, because tetrazoles start to spontaneously degrade at 110˜120° C., the ring opening reaction includes the highly likely risk of a runaway reaction.
In terms of a selection of reaction, as there are two reaction sites in each (R)-2-aryl-oxirane and tetrazole, the ring-opening reaction therebetween affords the substitution of 1N- or 2N-tetrazole at the benzyl or terminal position, resulting in a mixture of a total of 4 positional isomers. Therefore, individual positional isomers are low in production yield and difficult to isolate and purify.